Apparatus and method for water treatment by adsorption

ABSTRACT

The present invention relates to an apparatus and method for removal of arsenic or other metal salts from drinking water. The invention uses a bed of granulated ferric hydroxide to adsorp the metal in a pressurized adsorption chamber. The bed is sized to about 10 m 3  and run with an EBCT of about 3 minutes which has been found to give an unprecedently long bed life of upto 200,000 bed volumes of treated water.

This application is submitted under 35 USC 371 as the national stageapplication of International application number: PCT/GB01/00822 filedFeb. 26, 2001 claiming priority to GB 0004579.9 filed Feb. 25, 2000.

FIELD OF INVENTION

The present invention relates to a method and apparatus for arsenic orother metal removal from water, in particular but not exclusively,potable ground water.

BACKGROUND

Ground water represents an important source of drinking water but hasbeen found to contain dissolved metal ions such as arsenic, copper,nickel, chromium, lead, cadmium, molybdenum, silver, mercury andmanganese, often at undesirable levels.

For example, recent regulations have stipulated that the level ofarsenic should be less than 10 μg/l. Arsenic and other metals may beremoved from water by using one or more of the following methods; (i)adsorption by activated aluminium, (ii) nanofiltration, (iii) in aclarification/filtration plant using overdosing coagulation; or (iv) ionexchange. Each of these processes has disadvantages. Activated aluminiumhas a limited adsorption capacity. Nanofiltration allows comparativelyslow processing rates. The construction of a coagulation plant has ahigh capital cost due to e.g. the large amount of land required and highoperating costs.

SUMMARY

The present invention seeks to provide an advantageous apparatus andmethod for the removal of metals.

According to a first aspect of the present invention there is providedan apparatus for removing metals from water comprising an adsorptionchamber including a bed having a bed height and comprising a ferricmaterial, a water inlet connectable to a first water supply on one sideof the bed, a water outlet on the other side of the bed and a backwashinlet connectable to a second water supply on the said other side of thebed, the apparatus being arranged for normal and backwash operations, innormal operation arsenic is adsorbed onto the bed from water flowingfrom the said one side to said other side, and in backwash operationwater flows from the backwash inlet to said one side of the bed, the bedis between 0.5 m and 2.0 m, preferably at least 1.0 m, and the empty bedcontact time (EBCT) is between 1 and 6 minutes.

According to a second aspect of the present invention there is provideda process for removing metal from water comprising the steps of:

-   -   a) providing a bed of ferric material having a bed height of        between 0.5 and 2.0 m;    -   b) in normal operation supplying water from a first water supply        from one side of the bed to the other side of the bed such that        arsenic is adsorbed from the water onto the bed;    -   c) in backwash operation supplying water from a second water        supply from said other side of the bed to said one side of the        bed so as to remove contaminating material from the bed without        substantially disrupting the media; wherein in normal operation,        the empty bed contact time for said water is between 1 and 6        minutes.

The present inventors have found that using a bed of ferric adsorptionmedia allows increased metal adsorption when compared to activatedaluminium (wt/wt) with the consequential reduction in the costassociated therewith. Surprisingly, given the fragile nature of the bed,a long interval between backwashing cycles is possible without clearlyincreasing the differential pressure across the bed and/or withoutsignificant reduction in the adsorption rate and capacity of the bed.Furthermore the bed of adsorption material required by the presentinvention is made from a material which is relatively fragile in natureand so has been proportioned to allow the bed to be formed by eductingthe material into the adsorber without the bed material being disrupted.Yet the bed maintains sufficient size to allow the treatment ofcommercial volumes of water. Surprisingly, it has been found that theadsorption bed of the invention can treat an unprecedently large volumeof water before replacement i.e. when it reaches the allowed/set limitfor the metal being removed from the water, e.g. in the case of arsenicthe treated water having 10 μg/l arsenic. The bed size in conjunctionwith the EBCT provide an adsorber with a high capacity, yet the bed sizeis not so great to cause logistical problems in maintaining the bed.This is important as the apparatus may well be in a remote locationwhere complicated and/or frequent maintenance would greatly increasecosts for the process. The adsorption bed of the present inventionshould not need to be replaced more frequently than once per annum whichis considered to be commercially acceptable given the cost of thematerial.

Preferred features of the invention have been given in the dependentclaims. The preferred features have each been found to improve the costefficiency of the plant and may be used together or individually.

The use of a ferric adsorption material was also found to allowrelatively high rates of metal adsorption and a reduction in the capitalexpenditure required to build a plant when compared to a sludge plant.

Ferric oxide and hydroxide containing natural materials have been foundto be both relatively cheap to use and to give satisfactory adsorptionefficiencies e.g. the ability to adsorb large quantities of arsenic,typically 1.3 g to 3.5 g of arsenic is removed per Kg of FerricHydroxide. The use natural material containing these two compounds hasbeen found to give high economy and high adsorption.

Advantageously according to the first and second aspects of the presentinvention a layer of gravel separates the media from the support base.Gravel has been found to be an economic inert material to use which iswidely available.

Advantageously according to the first, second and third aspects of theinvention the pH use is in the range of 6.5 to 8.5, particularly 7.0 to8.0 and more particularly, 7.8 during normal operation. The adsorptionof arsenic is satisfactory within this PH range and normally this meansthat no adjustment to the PH is required for processing greatly reducingthe cost and complexity of the operating condition which latteradvantage is very important given the plant is normally unmanned.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the accompanyingdrawings, in which:

FIG. 1 is a schematic illustration of a arsenic adsorption plantaccording to the present invention;

FIG. 2 shows a vertical cross sectional view through an adsorberaccording to FIG. 1; and

FIG. 3 shows a horizontal cross sectional view of an adsorber accordingto FIG. 1.

DETAILED DESCRIPTION OF DRAWINGS

The present invention is exemplified with the following description anddrawings which show an arsenic adsorption plant. The invention is notlimited to arsenic adsorption, but this is the preferred embodiment ofthe invention. The metal can be selected from the group consisting ofarsenic; copper; cadmium, nickel; chromium; silver; lead; molybdenum;manganese; and mixtures thereof.

FIG. 1 shows a plant according to the present invention which has fiveadsorbers 10 operating in parallel. Although the present invention canfunction with a single adsorber it is preferred, the reasons which willbecome apparent below, to have a plurality of adsorbers 10.

Water to be treated flows from water inlet 12 through inlet line 14 tothe inlet 13 of each of the adsorbers 10. The water supplied normallycomes from a ground water source. The inlet line 14 has an inlet branch16 to each of the adsorbers 10. Each inlet branch 16 includes an inletcontrol valve 18 which is controllable to allow water flow into theparticular adsorber 10 to which respective branch 16 extends. The waterinlet 12 may be diverted around the plant via a dedicated bypass line.

In the illustrated embodiment the water inlet 12 is supplied from amixing tank (not shown) upstream of the water inlet 12.

Each inlet control valve 18 is arranged to allow a predetermined rate ofwater flow into the respective adsorber 10. Water flow through inletbranch 16 will be prevented in certain instances, for example duringbackwashing cycles of the adsorber 10 and commissioning cycles of theadsorber 10 as explained in more detail below. In other cases the inletcontrol valve 18 is adjusted to provide a proportion of the water to betreated into the adsorber 10.

Each adsorber 10 has a water outlet 20 opening into an outlet line 22via a respective outlet branch line 24. Each outlet branch line 24 isprovided with an outlet branch valve 25 between the adsorber outlet 20and the outlet line 22.

The illustrated plant is also provided with backwash and conditioningwash cycles for backwashing and conditioning washes of the adsorber 10.In the illustrated embodiment the water is arranged to flowsubstantially in the reverse direction through each adsorber 10 inbackwashing cycles. In the illustrated embodiment, the inlets for thebackwashing and conditioning washes are formed from the adsorbersoutlets 20 and the outlet for the backwashing and conditioning water isformed through the water inlet 13 in the adsorber 10. Thus, theinlet/outlet 13,20 in the adsorber 10 perform multiple functions andthis is advantageous in reducing the required ports into the adsorber10. This is preferred in the present invention though of course separateinlet/outlets could be provided for in each washing cycle or thebackwashing and conditioning washes may have a separate inlet andoutlets compared to the normal wash cycles which may be advantageouswhere the flow rate and volumes between backwashing and normal operationare particularly different.

In the illustrated embodiment the backwash inlet line 30 is fed frombackwash pumps 32. The backwash line 30 has backwash inlet line branches34 to each adsorber 10. The backwash line 30 is also used forconditioning washes.

Each backwash inlet line branch 34 opens into the respective outletbranch line 24 between the adsorber outlet 20 and the outlet branch linevalve 25.

The backwash inlet line branch 34 has a backwash inlet line valve 36immediately before opening into the outlet line branch 24. This valve 36prevents water flow through the backwash when the inlet line branch 34is in normal operating cycle; and, as will be apparent operates inconjunction with outlet line branch valve 25 to allow water flow throughthe backwash line 30, 34 in backwashing and in conditioning cycleswhilst at the same time valve 25 prevents flow through outlet branchline 24 to the outlet line 22.

In a backwashing cycle and conditioning wash cycles, water is outletfrom the adsorber 10 through the water inlet 13. A backwash-outlet line38 is linked to each adsorber via backwash outlet line branch 37. Theinlet control valve 18 is provided in the inlet line branch 16 which isclosed during backwashing and conditioning cycles to prevent water flowtowards the water inlet 12. A corresponding backwash outlet branch valve39 is provided in the backwash-outlet branch 37 which prevents flowthrough this line 37 during normal cycles of the apparatus, but allowswater flow through this line 37 during backwash and conditioning washcycles. The inlet line branch 16 opens into the backwash outlet branchline 37 between the valve 19 and the inlet 13. The apparatus is arrangedfor backwash operation having a flow rate of less than 50 m/h,preferably less than 30 m/h.

Each backwash-outlet line branch 37 opens into the backwash outlet line38 which feeds either into a holding tank 40 or directly to a wasteoutlet, e.g. a sewer 42. In backwashing cycles, the apparatus isnormally designed to feed into the holding tank 40 which is provided asa settlement tank to allow particulate matter washed out of theadsorbers 10 to settle. The tank 40 is provided with a top waterdraw-off device and the cleaned water from which outputs to the wateroutlet 42. A pump can be provided to pump the water from out of theholding tank 40 if desired. Preferably, a forward flush immediatelyprecedes a backwash operation. Tthe apparatus is structured such thatthe forward flush comprises less then about 10 bed volumes of water andcomprises flushing water from the second supply from the said one sideof the bed to the said other side of the bed. The height of the bed isbetween 0.5 m and 2.0 m, preferably at least 1.0 m. Preferably, theforward flush has a flow rate of less than 30 m/h, more preferably, theforward flush has a flow rate of less than 25 m/h.

In the conditioning washing cycles of the apparatus, the conditioningwash waters are directed directly to the outlet 42 through a filterarrangement 44. In conditioning washes, considerably more particularmatter is washed out of the adsorbers and the use of the filtercartridges 46 therefore removes this particulate matter before the wateris outlet to waste. There are of course requirements on the quality ofwater allowed to pass to waste. Use of the cartridge filter then meansthat the cartridge can then be removed after a conditioning cycle anddisposed of separately. It has an advantage in so far as the holdingtank volume can be greatly reduced if it does not have to be used forconditioning washes as well as back washes. Preferably, the conditioningcycle comprises an initial bed stratification backwash, a finalconditioning backwash and a final conditioning forward flush. In anotheraspect, the initial bed stratification backwash comprises about 4 bedvolumes of water at about 30 m/h.

Exploded cross-sections of the adsorbers 10 are shown in FIG. 2. FromFIG. 2 it is clear that in the illustrated embodiment the adsorber 10comprises a substantially cylindrical casket with domed closures at thetop and bottom. The adsorber 10 has an inlet 13 and an outlet 20 throughwhich water is pumped through the chamber. In normal cycle, water entersthrough the inlet 13, passes down the chamber to the outlet 20. Inbackwashing and conditioning cycles water mainly through the outlet 20and exits via the inlet 13, though of course in forward flush modeswater travels in the same direction as the normal cycle. The adsorber 10includes a base 62 which supports a media column 64. The base is locatedabove the outlet 20. The column 64 extends from the base 62 to justbelow the inlet 13. The media column 64 comprises a thin layer of gravel68 over which particulate adsorption media 66 rests. The adsorptionmedia 66 must adsorb arsenic thereonto from water.

In the illustrated embodiment the adsorption media 66 is a particulateferric oxide material produced from a naturally mined ore. This has beentreated to have an average diameter of less than 2 mm and more than 0.25mm. In the illustrated embodiment, less than 10% of the granules have adiameter greater than 2 mm and less than 5% of the granules have adiameter of less than 0.25 mm. The average diameter is approximately 0.8mm. In the granulization process of the ferric material, the graindensity is adjusted to between 1 and 2 kg/dm³ and is preferably within arange of about 1 .kg/dm³ to 1.7 kg/dm³ and, more preferably in theregion of 1.58 kg/dm³. The average bulk density at 45% water contentshould be above 1.1 g/cm³, but no more than 1.4 g/cm³. The bulk densityin the illustrated embodiment is controlled to 1.25 g/cm³. The granulesare controlled to have a particular surface about 1.6×10⁻⁵ m²/dm³. Thenatural material contains at least 50% by weight iron. The granulatedferric material of this nature has a very high adsorption of arsenicwhich allows unprecedently long bed lifes. Also through the control ofthe properties as described above, the material is substantially morerobust than untreated material which allows it to be handled and used.The untreated material has been found to smash when pouring intoadsorbers and can not form a useable media bed.

Alternatively the granulated ferric material is formed synthetically.The synthetic material will be treated to produce similar robustness andsurface features to allow the high adsorption and handability of thenaturally formed material. The naturally produced material is slightlypreferred at present in view of the cost of a naturally producedmaterial compared to a synthetic material. However, it is expected thatsynthetic manufacturing processes will be able to produce a syntheticmaterial at an acceptable cost for commercial situations, even thoughthey may be about 20% also more expensive than a naturally formedmaterial. The advantages of a synthetic material are that the adsorptioncapacity may be even higher than a naturally produced material in viewof the purity of the synthetic constituents. Table 1 shows theconstituents of a preferred synthetic material which may be used as theadsorption media 66. One preferred process comprises the step ofreplacing the ferric material after at least 120,000 bed volumes ofwater have been treated in normal operation.

The gravel base layer 68 is put into the media column on the base 62 inorder to prevent the granulated material 66 being washed from theadsorber 10 through the base 62. The base 62 must be provided withthrough openings 63 in order to allow backwashing flow and thus thesmall granular size ferric material may be washed through these openings63. This immediate layer 68 of gravel prevents such loss. Of course,other supports could be used instead of gravel. The base 62 may beprovided with nozzle openings 63 on a platform or alternatively maycomprise a header and lateral pipework arrangement fitted with eithermesh sleeves or nozzles.

Turning now to FIG. 2, it will be clear that there are several portsinto the adsorber 10. There is a manway 72 located about half way downthe chamber. This is provided to allow access to the interior of thechamber. There are sight glasses 74, 76 located to allow visualinspection of the interior of the chamber. Main port 78 is provided toallow the media to be poured into the chamber from the top thereof.There is also a main outlet port 80 provided to allow access to theunderside of the nozzle plate, next thereto is a smaller port 82 to actas a drain outlet.

Above the main inlet port 78 a davit 90 is provided which is used whenfilling the chamber with the media columns 64.

The adsorber 10 is supported on four legs 88.

Each of the adsorbers 10 are substantially the same as in theillustrated embodiment, though sizes and detailed construction may varyin particular installations.

It is preferred to have a plurality of adsorbers, rather than a singlelarger adsorber. The plurality of adsorbers allows one of the adsorbersto be taken offline, e.g. for backwashing and refilling of media column64, whilst the other adsorbers take the flow which would normally beprocessed by the adsorber taken out of line.

The present invention relates to an apparatus and method for removal ofarsenic or other metal salts from drinking water. The invention uses abed of granulated ferric hydroxide to adsorp the metal in a pressurizedadsorption chamber. The bed has a bed volume capacity of at least 10,000bed volumes of water to be treated during normal operations.Alternatively, the bed volume is at least 30,000 bed volumes andpreferably, 60,000 bed volumes of water to be treated during normaloperations between backwash operations. During backwash operations, thebed has a bed volume capacity ranging from about 5 bed volumes of waterto about 10 bed volumes of water.

The bed is sized to about 10 m³ and run with an EBCT of about 3 minuteswhich has been found to give an unprecedently long bed life of upto200,000 bed volumes of treated water.

1. An apparatus for removing metals from water comprising: an adsorptionchamber including a bed adapted to absorb metals, the bed having a bedheight ranging from about 0.5 m to about 2.0 m, the bed comprising aferric material, the ferric material comprising a granular ferricmaterial having a grain density ranging from about 1 kg/dm³ to about 2kg/dm³; a water inlet connectable to a first water supply on one side ofthe bed; a water outlet on the other side of the bed; a backwash inletconnectable to a second water supply on the said other side of the bed;wherein the apparatus is structured for normal operations and backwashoperations, during normal operations, the apparatus is adapted forabsorption of metals onto the bed as water flows from said one side tosaid other side, and the apparatus is structured for backwash operationsas water flows from the backwash inlet to said one side of the bed andthe empty bed contact time (EBCT) is between 1 and 6 minutes.
 2. Theapparatus according to claim 1, wherein the bed has a bed volumecapacity of at least 10,000 bed volumes of water to be treated duringnormal operations.
 3. The apparatus according to claim 1, wherein thebed has a bed volume capacity of at least 60,000 bed volume of water tobe treated during normal operations.
 4. The apparatus according to claim1, wherein the bed has a bed volume capacity of at least 30,000 bedvolume of water to be treated during normal operations between backwashoperations.
 5. The apparatus according to claim 1, wherein the bed has abed volume capacity ranging from about 5 bed volumes of water to about10 bed volumes of water during backwash operations.
 6. The apparatusaccording to claim 1 wherein the apparatus is arranged for backwashoperation having a flow rate of less than 50 m/h.
 7. The apparatusaccording to claim 1 wherein the bed is arranged for backwash operationhaving a flow rate of less than 30 m/h.
 8. The apparatus according toclaim 1 wherein the apparatus is structured such that a forward flushimmediately precedes a backwash operation.
 9. The apparatus according toclaim 8, wherein the apparatus is structured such that the forward flushcomprises less then about 10 bed volumes of water.
 10. The apparatusaccording to claim 8 wherein the apparatus is structured such that theforward flush comprises flushing water from the second supply from thesaid one side of the bed to the said other side of the bed.
 11. Theapparatus according to claim 8, wherein the apparatus is structured suchthat the forward flush has a flow rate of less than 30 m/h.
 12. Theapparatus according to claim 8, wherein the apparatus is structured suchthat the forward flush has a flow rate of less than 25 m/h.
 13. Theapparatus according to claim 1 wherein the adsorption chamber furthercomprises a base arranged to support the bed.
 14. The apparatusaccording to claim 13, wherein the base includes one or more openings.15. The apparatus according to claim 9 wherein the base comprises awedgewire material.
 16. The apparatus according to claim 1 wherein thegranular ferric material has an average diameter of less than 2 mm andmore than 0.25 mm, 10% or less of the granules have a diameter greaterthan 2 mm and 5% or less of the granules have a diameter less than 0.25mm.
 17. The apparatus according to claim 1, wherein the granular ferricmaterial has an average diameter of about 0.8 mm.
 18. The apparatusaccording to claim 1 wherein the granular ferric material is agranulated natural material.
 19. The apparatus according to claim 1wherein the granular ferric material comprises ferric hydroxide.
 20. Theapparatus according to claim 1 wherein the grain density ranges fromabout 1.5 kg/dm³ to about 1.7 kg/dm³.
 21. The apparatus according toclaim 1 wherein the grain density is about 1.58 kg/dm³.
 22. Theapparatus according to claim 21 wherein the average bulk density at 45%water content is at least 1.1 g/cm³ and no more than 1.4 g/cm³.
 23. Theapparatus according to claim 21 wherein the average bulk density at 45%water content is within a range of about 1.2 g/cm³ to 1.3 g/cm³.
 24. Theapparatus according to claim 21 wherein the average bulk density at 45%water content is approximately 1.25 g/cm³.
 25. The apparatus accordingto claim 1 wherein the material has a specific surface of about 1.6×10⁵m²/dm³.
 26. The apparatus according to claim 1 wherein the materialcomprises at least 50% by weight iron.
 27. The apparatus according toclaim 1 wherein the bed includes a layer of gravel of greater averagediameter than the ferric material.
 28. The apparatus according to claim1 where the metal is selected from the group consisting of arsenic;copper; cadmium, nickel; chromium; silver; lead; molybdenum; manganese;and mixtures thereof.
 29. The apparatus according to claim 1 wherein thebed has a bed volume and the bed is structured such that the ferricmaterial is replaced after at least 100,000 bed volumes of water havebeen treated in normal operation.
 30. The apparatus according to claim 1wherein the bed has a bed volume and the bed is structured such that theferric material is replaced after at least 120,000 bed volumes of waterhave been treated in normal operation.
 31. The apparatus according toclaim 1 wherein the bed has a bed volume and the bed is structured suchthat the ferric material is replaced after at least 150,000 bed volumesof water have been treated in normal operation.
 32. The apparatusaccording to claim 1 wherein the pH in the bed of the adsorption chamberis within the range of 7.0 and 8.5 during normal operation.
 33. Theapparatus according to claim 1 wherein the pH in the bed of theadsorption chamber is within the range of 7.5 and 8.0 during normaloperation.
 34. The apparatus according to claim 1 wherein the pH in thebed of the adsorption chamber is within the range of 7.8 during normaloperation.
 35. The apparatus according to claim 1 wherein the bed is atleast 1 m in height and the bed has a volume of about 10 m³.
 36. Theapparatus according to claim 1 wherein the apparatus is arranged toprovide a conditioning cycle when new bed material is added to theadsorption chamber, wherein the conditioning cycle comprises an initialbed stratification backwash, a conditioning backwash and a finalconditioning forward flush.
 37. The apparatus according to claim 36wherein the initial bed stratification backwash comprises about 4 bedvolumes of water at about 30 m/h.
 38. The apparatus according to claim36 wherein the final conditioning backwash comprises a backwash cycle asdefined in the preceding claims.
 39. The apparatus according to claim 36wherein the conditioning forward flush comprises a forward flush asdefined in the preceding claims.
 40. The apparatus according to claim 1wherein the apparatus is arranged to provide a loading backwash of about10 bed volumes of water at less than 10 m/h e.g. about 5 m/h, whenloading the bed material into the adsorption chamber.
 41. The apparatusaccording to claim 1 wherein the apparatus includes a plurality ofsimilar adsorption chambers having shared first water supplies andsecond water supplies.
 42. The apparatus of claim 41 wherein one of theadsorption chambers is undergoing any backwash cycle or forward flush atany one time.
 43. The apparatus according to claim 41 wherein theadsorption chambers comprise a first group and a second group and thefirst water supply is arranged to supply water at different rates to thefirst and second groups.
 44. The apparatus according to claim 1 whereinthe bed is structured such that the rate of water through the bed innormal operation is less than 50 m/h.
 45. The apparatus according toclaim 1 wherein the bed is structured such that the rate of waterthrough the bed in normal operation is about 25 m/h.
 46. The apparatusaccording to claim 1 wherein the EBCT in normal operation is about 3minutes.
 47. An apparatus for removing metals from water comprising: anadsorption chamber including a bed adapted to absorb arsenic, the bedhaving a bed height ranging from about 0.5 m to about 2.0 m, the bedcomprising a ferric material, the ferric material comprising a granularferric material having a grain density ranging from about 1 kg/dm³ toabout 2 kg/dm³, the bed comprising a bed volume capacity of at least60,000 bed volumes of water to be treated during normal operationsbetween backwash operations; a water inlet connectable to a first watersupply on one side of the bed; a water outlet on the other side of thebed; a backwash inlet connectable to a second water supply on the saidother side of the bed; wherein the apparatus is structured for normaloperations and backwash operations, during normal operations, theapparatus is adapted for absorption of metals onto the bed as waterflows from said one side to said other side, and the apparatus isstructured for backwash operations as water flows from the backwashinlet to said one side of the bed and the empty bed contact time (EBCT)is between 1 and 6 minutes.wherein the.
 48. A process for removing metalfrom water comprising: a) providing a bed of ferric material having abed height of between 0.5 and 2.0 m wherein the ferric materialcomprises a granular ferric material having a grain density between 1 to2 kg/dm³; b) in normal operation, supplying water from a first watersupply from one side of the bed to the other side of the bed such thatmetal is adsorbed from the water onto the bed; c) in backwash operationsupplying water from a second water supply from said other side of thebed to said one side of the bed so as to remove contaminating materialfrom the bed without substantially disrupting the media; wherein innormal operation, the empty bed contact time for said water is between 1and 6 minutes.
 49. The process according to claim 48 wherein at least10,000 bed volumes of water are treated in normal operation betweenbackwash operations.
 50. The process according to claim 48 wherein atleast 30,000 bed volumes of water are treated in normal operationbetween backwash operations.
 51. The process according to claim 48wherein 60,000 bed volumes of water are treated in normal operationbetween backwash operations.
 52. The process according to claim 48wherein the backwash operation comprises about 10 bed volumes of water.53. The process according to claim 48 wherein the backwash operationcomprises about 5 bed volumes of water.
 54. The process according toclaim 48 wherein the backwash operation has a flow rate of less than 50m/h.
 55. The process according to claim 48 wherein the backwashoperation has a flow rate of less than 30 m/h.
 56. The process accordingto claim 48 wherein a forward flush immediately precedes a backwashoperation.
 57. The process according to claim 56 wherein the forwardflush comprises about 10 bed volumes of water.
 58. The process accordingto claim 56 wherein the forward flush comprises about 5 bed volumes ofwater.
 59. The process according to claim 56 wherein the forward flushcomprises flushing water from the second supply from the said one sideof the bed to the said other side of the bed.
 60. The process accordingto claim 56 wherein the forward flush has a flow rate of less than 30m/h.
 61. The process according to claim 56 wherein the forward flush hasa flow rate of less than 25 m/h.
 62. The process according to claim 48,wherein the bed is supported on a base.
 63. The process according toclaim 62 wherein the base includes openings.
 64. The process accordingto claim 62, wherein the base comprises a wedgewire material.
 65. Theprocess according to claim 48, wherein the granular ferric material hasan average diameter of less than 2 mm and more than 0.25 mm, less than10% of the granules have a diameter greater than 2 mm and less than 5%of the granules have a diameter less than 0.25 mm.
 66. The processaccording to claim 48 wherein the granular ferric material has anaverage diameter of about 0.8 mm.
 67. The process according to claim 48wherein the granular ferric material is a granulated natural material.68. The process according to claim 48 wherein the material comprisesferric hydroxide.
 69. The process according to claim 48 wherein thegrain density is between 1.5 to 1.7 kg/dm³.
 70. The process according toclaim 69 wherein the grain density is about 1.58 kg/dm³.
 71. The processaccording to claim 69 wherein the average bulk density at 45% watercontent is at least 1.1 g/cm³ and/or no more than 1.4 g/cm³.
 72. Theprocess according to claim 69 wherein the average bulk density at 45%water content is within a range of from about 1.2 to about 1.3 g/cm³.73. The process according to claim 69 wherein the average bulk densityat 45% water content is about 1.25 g/cm³.
 74. The process according toclaim 48 wherein the granular ferric material has a specific surface ofabout 1.6×10⁵ m²/dm³.
 75. The process according to 48, wherein thegranular ferric material comprises at least 50% by weight iron.
 76. Theprocess according to claim 48, wherein the bed includes a layer ofgravel of greater average diameter than the ferric material.
 77. Theprocess according to claim 48 wherein the metal removed from the wateris selected from the group consisting of: arsenic; copper; cadmium;nickel; chromium; silver; lead molybdenum and mercury.
 78. The processaccording to claim 48 wherein the ferric material is replaced after atleast 100,000 bed volumes of water have been treated in normaloperation.
 79. The process according to claim 48 wherein the ferricmaterial is replaced after at least 120,000 bed volumes of water havebeen treated in normal operation.
 80. The process according to claim 48wherein the ferric material is replaced after at least 150,000 bedvolumes of water have been treated in normal operation.
 81. The processaccording to claim 48 wherein the pH in the bed of the adsorptionchamber is normal operation is within a range of about 7.0 to about 8.5.82. The process according to claim 48 wherein the pH in the bed of theadsorption chamber is normal operation is within a range of about 7.5 to8.0.
 83. The process according to claim 48 wherein the pH in the bed ofthe adsorption chamber is normal operation is about 7.8.
 84. The processaccording to claim 48, wherein the bed height is at least 1 m.
 85. Theprocess according to claim 48 wherein the process further comprises aconditioning cycle when new bed material is added to process, whereinthe conditioning cycle comprises an initial bed stratification backwash,a final conditioning backwash and a final conditioning forward flush.86. The process according to claim 85 wherein the initial bedstratification backwash comprises about four bed volumes of water atabout 30 m/h.
 87. The process according to claim 85 wherein the finalconditioning backwash comprises a backwash cycle as defined in any ofthe preceding claims.
 88. The process according to claim 85 wherein theconditioning forward flush comprises a forward flush as herein beforedefined in any of the preceding claims.
 89. A process for removingarsenic from water comprising: a) providing a bed of ferric materialhaving a bed height of between 0.5 and 2.0 m wherein the ferric materialcomprises a granular ferric material having a grain density between 1 to2 kg/dm³; b) in normal operation, supplying water from a first watersupply from one side of the bed to the other side of the bed such thatmetal is adsorbed from the water onto the bed wherein 60,000 bed volumesof water are treated in normal operation between backwash operations; c)in backwash operation supplying water from a second water supply fromsaid other side of the bed to said one side of the bed so as to removecontaminating material from the bed without substantially disrupting themedia; wherein in normal operation, the empty bed contact time for saidwater is between 1 and 6 minutes; (d) replacing the ferric materialafter at least 120,000 bed volumes of water have been treated in normaloperation.
 90. A process for removing metal from water comprising: a)providing a bed of ferric material having a bed height of between 0.5and 2.0 m wherein the ferric material comprises a granular ferricmaterial having a grain density between 1 to 2 kg/dm³; b) in normaloperation, supplying water from a first water supply from one side ofthe bed to the other side of the bed such that metal is adsorbed fromthe water onto the bed wherein 60,000 bed volumes of water are treatedin normal operation between backwash operations; c) in backwashoperation supplying water from a second water supply from said otherside of the bed to said one side of the bed so as to removecontaminating material from the bed without substantially disrupting themedia; wherein in normal operation, the empty bed contact time for saidwater is between 1 and 6 minutes; (d) replacing the ferric materialafter at least 120,000 bed volumes of water have been treated in normaloperation; (e) performing a conditioning cycle when new bed material isadded to process, wherein the conditioning cycle comprises an initialbed stratification backwash, a final conditioning backwash and a finalconditioning forward flush.